Cast Silver Hinge Becomes Unhinged

My husband Frank has come a long way since the days he’d buy a new pair of eyeglasses only when the tape wouldn’t hold the old ones together any longer.

A materials scientist, I think he likes shopping for and buying new glasses because he’s intrigued by the new frame designs and materials designers are using. So it isn’t surprising that he chose a Chrome Hearts frame for his new prescription sunglasses. One of the key selling points of this not-inexpensive brand is the novel material combinations used, including sterling silver, titanium, platinum, 22K gold, wood -even diamonds if you can believe it.

The hinge on this particular frame consists of two interleaving circular halves (cast parts) that hold the screw/pin. Unfortunately within two weeks, one half of the hinge had failed-catastrophically. (Note that the parts of the hinge that broke off are still embedded in the other half of the hinge on the glasses side.)

Just in case you’re wondering, Frank wasn’t bungie-jumping or playing hockey in these specs. As I recall, he was mostly lying on a deck chair reading a book, though he might debate that.

In its marketing materials, the manufacturer claims that the hinge is made of .925 sterling silver. They don’t state what the alloy is, though a quick check of the web indicates that most sterling silver jewelry is 92.5% silver and 7.5% copper, which makes sense since pure silver wouldn’t hold its shape for any time at all.

Either the wrong alloy was used, or it’s one heck of a manufacturing flaw for a casting to break like this did. I would be interested in hearing what you readers have to say about the relative compliance of the quite robust plastic components of this frame and what appears to be a relatively small (and, thus weak) temple hinge.

The good news is that the store he purchased the frame from recognized there was no possible way they could try and fix the part and gave him a full refund.

For further enlightenment, I contacted Ken Russell, who was one of Frank’s materials science professors at MIT and a forensic engineer. Besides taking Frank to task for forgetting some basic principles, here’s what Ken had to say:

“Your posting on the defective glass frame hinge saddens this old metallurgist’s heart. I taught (or tried to teach) your husband the principles of metallurgy in class many years ago. The class even took a bit of a holistic approach long before the word became popular.

He still remembers learning of a “blivet,” which is ten pounds of excrement in a five pound bag. Yet he purchased these glass frames, constructed of sterling silver, titanium, platinum, 22K gold, and even wood. No reputable mechanical designer would use such a mishmash of materials. This wretched excess boggles the imagination and exemplifies the blivet.

It is surprising that the silver failed first. I would suspect that the wood would rot or be consumed by termites.

The ‘92.5′ does indicate the purity of sterling silver. The 7.5% copper strengthens the silver and of course reduces the price of the final alloy. So why not put in more copper and reduce the price further? The silver will only dissolve 7.5% copper. Add any more and you get fine copper particles and a coppery look. The situation is rather like precipitating salt out of a concentrated brine solution.

Sterling silver is usually drawn, rolled, or stamped to final form. This cold work not only strengthens the alloy but tends to ameliorate casting defects. The hinge in question was apparently cast to final shape. Not only was the alloy weak, it might well have had inclusions, porosity, cracks or other defects that bedevil casting.

I am not picking a fight with the casting industry. Mostly they do a good job. But, producing a quality casting demands careful process design and control and meticulous quality assurance. Perhaps even more important the foundry has to know the demands on the cast part. An eyeglass hinge takes much more of a beating than say, the hinge clip on a brooch. I suspect that one or more of those factors was missing in the case of the hinge.”

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